The present invention relates to a matrix liquid crystal display, more particularly, to the drive circuit of a matrix liquid crystal device incorporating switching transistors that are provided for each display picture element.
Conventionally, such a liquid crystal display device containing switching transistors directly connected to each display picture element set in the matrix liquid crystal display panel with a liquid crystal layer inserted between the display picture element electrodes and the opposite electrodes can achieve a high-quality display even when a multiplex drive is performed via multiple lines. Typically, such a configuration is shown as the equivalent circuit of FIG. 1. In FIG. 1, reference number 11 indicates switching transistors made of thinly filmed semiconductive material, being positioned at the crossing of the row electrodes 12 and the column electrodes 13. Each unit of the row electrode 12 and column electrode 13 is connected to a gate terminal and a source terminal, respectively. Reference number 14 indicates the liquid crystal layer capacitors each being inserted between the display picture element electrode connected to the drain terminal of the switching transistor 11 and the opposite electrode 15. However, if a capacitor storing charge is provided in parallel with the liquid crystal layer, reference number 14 will become the parallel capacitor for the liquid crystal layer and for storing charge as well.
In reference to the drive waveforms shown in FIG. 2, principles of the operation of the above liquid crystal display unit are described below. FIG. 2 (a) shows one of the scan pulses being applied to each row electrode 12, where the pulse sequentially activating each row of the switching transistor 11 being connected to each row electrode 12 is delivered to the switching transistor 11 via each row electrode 12. FIG. 2 (b) shows one of the video signal waveforms being applied to each column electrode 13, where a specific voltage is applied to the source terminal of the switching transistor 11 in response to the depth of the display in each column. Since an AC voltage is fed to the liquid crystal layer, video signals become pulsive waveforms with inverted fields of alternate polarity. We'll now look at the picture elements located at the crossing of the row electrode 12 that received the scan pulse shown in FIG. 2 (a) and the column electrode 13 that received the video signal wave form shown in FIG. 2 (b).
First, as soon as the switching transistor 11 turns ON upon receipt of the scan pulse in the period "t1", the liquid crystal layer 14 is charged via the switching transistor 11, thus providing the display picture element electrode connected to the drain terminal with the voltage V which is exactly the same as that of the video signal being fed to the source terminal. During a period from "t1" to "t2", the switching transistor 11 remains OFF to retain charge in the liquid crystal layer 14, causing the display picture element electrode to also retain the voltage V without varying its potential while the potential of the opposite electrode is constant. When the switching transistor 11 turns ON in the period "t2", the display picture element electrode is then charged up to the -V potential. When the switching transistor 11 turns OFF, this voltage is retained by this electrode. As a result, a specific voltage waveform is generated in the display picture element electrode as shown in FIG. 2 (c), stabilizing the potential of the opposite electrode 15 at a specific level, for example, zero volt. As a result, a rectangular waveform of the effective voltage can be applied to the liquid crystal layer 14. As described above, even when such a multiplex drive system is used by the liquid crystal display device containing switching transistors, the liquid crystal layer 14 can still receive a stable voltage equivalent to the static drive shown in FIG. 2 (c), thus achieving a satisfactory display. FIG. 3 shows the block diagram of a typical drive circuit of a liquid crystal display device containing switching transistors. In FIG. 3, reference number 21 indicates the liquid crystal display panel. Reference number 22 indicates a row electrode driver that outputs scan pulses to each row electrode. Reference number 23 indicates a column electrode driver that converts video signals that arrive at each column in series into parallel signals and simultaneously outputs these to each column electrode. Reference number 24 indicates a video signal processor circuit which, for example, converts TV broadcast video signals into specific waveforms suited for driving the liquid crystal. Reference numbers 25 and 26 respectively indicate amplifier circuits which, using switch 27, switch the inverted and non-inverted video signals before feeding these signals to the column electrode driver. Reference number 28 indicates the control circuit controlling these operations.
The power supplied to the liquid crystal display drive circuit is mainly consumed by analogue circuits such as the video signal processor circuit 24 and the amplifier circuits 25/26. Therefore, to effectively reduce the power consumption in the analogue circuits, the power voltage in these circuits should be lowered to the least level. Nevertheless, the amplitude of the video signal is dependent on the applicable voltage needed for properly driving the liquid crystals. In other words, since the amplitude can be determined by the display characteristics of the liquid crystals themselves, the existing drive system cannot lower the power voltage significantly nor achieve effective power savings. When using thin-film generating technology, to form the switching transistors on a transparent substrate with either a thin or thick film, temperatures may vary during the operation causing a variance in the physical characteristics of the made-up films. This effect is particularly troublesome when making up a large liquid crystal panel, since display contrast may become variable, too. To prevent this, the amplitude of the video signal amplifier circuit may be varied according to the varied characteristics of the transistors in order that variations may be properly compensated for. However, to provide the amplifier circuit with the corrective function, an extremely complex circuit is needed. Thus the problem competes with the goals of achieving compact size and minimum power consumption in these circuits.